This invention relates to echo suppressors for two-way transmission systems and, more particularly, to improved digital break-in circuitry for such echo suppressors.
Echoes are produced in a transmission system whenever an impedance discontinuity or mismatch exists, such as at the junction between a four-wire transmission facility and a two-wire transmission facility. The annoying effects of echoes can be minimized by the use of echo suppressor apparatus which essentially operates to disable the echo path of a subscriber when he is transmitting. Thus, basically, an echo suppressor is a voice-operated switching device which may be situated near one end of a four-wire transmission facility. If echoes in both directions are to be suppressed by the device, it is commonly referred to as a full echo suppressor; if echoes are suppressed in only one direction, the device is referred to as a split echo suppressor.
In a split echo suppressor, the echo suppressor apparatus nearest a particular subscriber end typically functions to disable the outgoing, or echo return, path from that subscriber when signals from the far-end subscriber appear on the incoming path. Thus, echoes due to incoming signals are prevented from returning to the far-end subscriber. Echo suppressor apparatus at the far subscriber end functions in a similar manner to prevent echoes from returning to the near-end subscriber when the near-end subscriber is transmitting.
During echo suppression in a system utilizing a split echo suppressor, suppression is removed from the outgoing path when the near-end subscriber breaks in, a condition commonly referred to as "double talking" since both subscribers are talking simultaneously. At the same time, the circuitry for effecting such break-in typically inserts a small fixed loss, on the order of 6-10 db, in the incoming path to reduce the level of echoes thereon during double talking. Unfortunately, the reduction of echoes using fixed losses in this manner is accompanied by a corresponding reduction in the level of speech signals received from the far-end subscriber and his message may become unintelligible, particularly if he speaks softly. Solutions to this problem in analog echo suppressor systems have been proposed using syllabic speech compressor and variolosser arrangements for providing a variable double-talking loss, as disclosed in P. T. Brady-G. K. Helder U.S. Pat. No. 3,305,646, issued Feb. 21, 1967 and G. K. Helder U.S. Pat. No. 3,313,893, issued Apr. 11, 1967.
In digital echo suppressors, voice and echo signals appear as digitally encoded words. A fixed loss may be provided during double talking, with the attendant limitations mentioned above, via connection of digital attenuator circuitry into the incoming path, as shown in R. E. LaMarche-C. J. May, Jr. U.S. Pat. No. 3,673,355, issued June 27, 1972. Alternatively, a digital implementation of a syllabic speech compressor might be employed in the manner of the above-mentioned Brady and Helder arrangements. However, straightforward implementation of a digital syllabic compressor for this purpose suffers from manifest disadvantages related to cost, speed and complexity.